1. Field of the Invention
The present invention relates generally to the drilling of oil and gas wells, and more particularly, to systems and methods for drilling well bores and evaluating subsurface zones of interest as the well bores are drilled into such zones.
2. Description of the Prior Art
During the drilling and completion of an oil and/or gas well, it is usually necessary to test and evaluate the production capabilities of the well. This is typically done by isolating a subsurface zone or formation of interest therein which is to be tested and subsequently flowing well fluid either into a sample chamber or up through a tubing string to the surface. Various data such as pressure and temperature of the produced well fluids may be monitored downhole to evaluate the long term production characteristics of the zone or formation.
One very commonly used well testing procedure is to first cement a casing in the well bore and then to perforate the casing adjacent one or more zones of interest. Subsequently, the well is flow tested through the perforations. Such flow tests are commonly performed with a drill stem test string which is a string of tubing located within the casing. The drill stem test string carries packers, tester valves, circulating valves and the like to control the flow of fluids through the drill stem test string.
Typical tests conducted with a drill stem test string are known as draw-down and build-up tests. For the "draw-down" portion of the test, the tester valve is opened and the well is allowed to flow up through the drill string until the formation pressure is drawn down to a minimum level. For the "build-up" portion of the test, the tester valve is closed and the formation pressure is allowed to build up below the tester valve to a maximum pressure. Such draw-down and build-up tests may take many days to complete.
There is a need for quick, reliable testing procedures which can be conducted at an early stage in the drilling of a well before casing has been set. This is desirable for a number of reasons. First, if the well is a commercially unsuccessful well, then the cost of casing the well can be avoided or minimized. Second, it is known that damage begins occurring to a subsurface producing zone or formation as soon as it is intersected by the drilled well bore, and thus, it is desirable to conduct testing at as early a stage as possible.
While techniques and systems have been developed for testing open, uncased well bores, it is often considered undesirable to flow test an open hole well through a drill stem test string from the standpoint of safety considerations. That is, the conduct of conventional draw-down and build-up testing in an open hole situation is dangerous in that the drill pipe is full of drilling mud which must be circulated out and it is possible for problems to occur such as blow-outs or differential pressure sticking of the pipe. It is preferable to conduct a test with a safe dead well which is completely kept under control due to the continuous presence of a column of heavy drilling mud therein.
One technique that has been used is to pull the drill pipe out of the well bore when it is desired to test a subterranean zone or formation penetrated by the well bore and to then run a special test string into the well for testing the zone or formation. This, of course, involves the time and cost of pulling and running pipe and is disadvantageous from that standpoint.
Thus, there is a need for integrated well drilling and testing systems and methods whereby subterranean zones of interest can be tested as the well bore is drilled into the zones to thereby quickly and inexpensively evaluate the production capability of the zones without substantially interrupting the drilling process.